The features of refractories, in which a carbonaceous bonding texture (carbon bond) is formed between particles of base materials, e.g., refractory inorganic oxide raw materials, such as alumina and magnesium, and carbon based raw materials, such as graphite, include that the wettability with slag is poor, the corrosion resistance is excellent, and the thermal shock resistance is also excellent because the thermal conductivity is high and the modulus of elasticity is low. In particular, these features are more pronounced when the refractory contains a carbon based raw material, e.g., graphite, as base material particles (hereafter referred to as “a carbon-containing refractory”). Therefore, the above-described refractories have been widely used as linings of hot metal transfer ladles and converters and refractories for continuous casting. Furthermore, in recent years, higher strength and higher thermal shock resistance have been required as the use condition of the refractory becomes severer.
With respect to the refractory, e.g., the above-described carbon-containing refractory, having a carbon bond, a technology for adding carbonaceous fibers to the material for the carbon-containing refractory has been developed as a method for improving the strength thereof (refer to Patent Documents 1 to 3). For example, Patent Document 1 discloses a magnesia carbonaceous brick in which carbonaceous fibers having outer diameters of 5 μm or less and lengths of 0.13 to 50 mm are mixed by dispersion. Patent Document 2 discloses that carbon fibers of 1 to 5 mm are added to a raw material formulation of a carbon-containing refractory. When carbonaceous fibers are added as described above, the carbonaceous fibers function as fillers and, thereby, the strength is increased and the corrosion resistance and the thermal shock resistance of the refractory can be improved. Patent Document 3 discloses carbon-containing refractory in which carbon fibers having outer diameters of 10 to 50 μm and lengths of 0.20 to 2 mm and a low-melting point active metal, e.g., Si or Al, are added to a refractory powder in order to improve poor affinity between the refractory material and the carbon fibers. According to this, in a heat treatment, the low-melting point active metal reacts with C and N in an atmosphere, protrusions composed of a non-oxidizing compound are formed on the surfaces of the carbon fibers and, thereby, the withdrawal resistance of the carbon fiber is increased and the joining effect is increased.
On the other hand, with respect to the carbon-containing refractory, the carbon component in the refractory is oxidized and eliminated during the use at high temperatures, decarburized portions become fragile, and dissolution loss and abrasion become remarkable. That is, a weak point of the carbon-containing refractory is the oxidation resistance at high temperatures, and there is a drawback in that the useful life is relatively short. Heretofore, for the purpose of improving the oxidation resistance, carbon-containing refractories, in which various oxidation resistance imparting agents have been added to the materials for carbon-containing refractories, have been developed.
For example, in Patent Document 4, metal powders, e.g., Al, B, Cr, Ti, and Mg, are used as the oxidation resistance imparting agents. According to this, an oxide of a metal powder is generated at high temperatures, gaps in the texture formed during molding are almost completely filled by volume expansion of the metal oxide so as to densify and, thereby, an increase in strength and a reduction in gas permeability are facilitated. Entrance of an oxidizing gas and slag into the texture is prevented by this densification and, in addition, the oxidation resistance is improved.
Patent Document 5 discloses that metal chromium or a chromium compound, e.g., chromium carbide or chromium boride, is added to a magnesia carbon brick. The metal chromium and the chromium compounds react with magnesia in a high temperature atmosphere so as to form MgO—Cr2O3 based high-melting point products. According to this, an apparent viscosity of the slag is increased, and elution of the magnesia aggregate into the slag is suppressed.
Patent Document 6 discloses a carbon-containing refractory formed by adding a metal alkoxide powder of Al, Ca, Mg, Zr, Si, Ti, Cr, or the like to graphite and a refractory raw material. The metal alkoxide decomposes at a high temperature of 300° C. or higher, and a part of alkoxide groups remain so as to strengthen the connection of a carbon bond. On the other hand, the metal portions react with CO, which is a main atmosphere in the inside of the refractory, so as to form metal carbides, and when nitrogen is contained, metal nitrides are formed. The texture is densified by volume expansion through formation of the metal carbides and the metal nitrides. Consequently, entrance of an oxidizing gas and slag into the texture is prevented and, in addition, the oxidation resistance is improved.
[Patent Document 1]    Japanese Examined Patent Application Publication No. 62-9553
[Patent Document 2]    Japanese Unexamined Patent Application Publication No. 3-90271
[Patent Document 3]    Japanese Unexamined Patent Application Publication No. 5-78180
[Patent Document 4]    Japanese Unexamined Patent Application Publication No. 54-163913
[Patent Document 5]    Japanese Unexamined Patent Application Publication No. 1-320262
[Patent Document 6]    Japanese Unexamined Patent Application Publication No. 6-64961
[Patent Document 7]    WO 00/40509
[Patent Document 8]    Japanese Unexamined Patent Application Publication No. 2002-293524
[Non-Patent Document 1]    Yahachi SAITO and Shunji BANDOW, “Ka-bon Nanochu-bu no Kiso (Introduction to Carbon Nanotubes)”, First Edition, CORONA PUBLISHING CO., LTD., Nov. 13, 1998, pp. 23-57.